Positive electrode active material for nonaqueous electrolyte secondary batteries, and production method thereof

ABSTRACT

A method for producing a positive electrode active material for nonaqueous electrolyte secondary batteries, includes: a mixing step of adding a W compound powder having a solubility A adjusted to 2.0 g/L or less to a Li-metal composite oxide powder and stirring in water washing of the composite oxide powder, the solubility A being determined by stirring the W compound in water having a pH of 12.5 at 25° C. for 20 minutes, the composite oxide powder being represented by the formula: LicNi1-x-yCoxMyO2 and composed of primary and secondary particles, followed by solid-liquid separation, to thereby obtain a tungsten-containing mixture with the tungsten compound dispersed in the composite oxide powder; and a heat-treating step of heat-treating the mixture to uniformly disperse W on the surface of primary particles and thereby form a compound containing W and Li from the W and Li in the mixture, on the surface of primary particles.

The present application is a divisional application of U.S. patentapplication Ser. No. 15/555,597, filed Sep. 5, 2017, the contents ofwhich are hereby incorporated by reference in their entirety.

BACKGROUND Field of the Invention

The present invention relates to a positive electrode active materialfor nonaqueous electrolyte secondary batteries and a production methodthereof.

Related Art

In recent years, with the wide adoption of portable electronic devicessuch as mobile phones and laptop computers, the development of small andlightweight nonaqueous electrolyte secondary batteries having highenergy density is strongly desired. Further, the development of highpower secondary batteries as batteries for electric cars includinghybrid cars is strongly desired.

Examples of the secondary batteries satisfying such demands includelithium ion secondary batteries. Such lithium ion secondary batteriesare composed of a negative electrode, a positive electrode, anelectrolyte, etc., and materials capable of intercalation anddeintercalation of lithium ions are used for the active materials of thenegative electrode and the positive electrode.

The lithium ion secondary batteries are now being actively studied anddeveloped. Above all, lithium ion secondary batteries using a layered orspinel lithium-metal composite oxide as a positive electrode materialallow a high voltage of 4-V class to be obtained, and therefore arebeing put into practical use as batteries having high energy density.

Main examples of materials proposed so far include lithium cobaltcomposite oxide (LiCoO2) that is comparatively easily synthesized,lithium nickel composite oxide (LiNiO2) using nickel that is lessexpensive than cobalt, lithium nickel cobalt manganese composite oxide(LiNi1/3Co1/3Mn1/3O2), and lithium manganese composite oxide (LiMn2O4)using manganese.

Among these, lithium nickel composite oxide and lithium nickel cobaltmanganese composite oxide are gaining attention as a material thatallows good cycle characteristics as well as low resistance and highpower to be obtained, and a resistance reduction that is necessary forpower enhancement is regarded as being important in recent years.

As a method for achieving the aforementioned resistance reduction,addition of different elements is conducted, and transition metalscapable of having high valence such as W, Mo, Nb, Ta, and Re areconsidered to be useful, in particular.

For example, Japanese Patent Laid-Open No. 2009-289726 proposes alithium transition metal compound powder for lithium secondary batterypositive electrode materials containing one or more elements selectedfrom Mo, W, Nb, Ta, and Re in an amount of 0.1 to 5 mol % with respectto the total molar amount of Mn, Ni, and Co, where the total atomicratio of Mo, W, Nb, Ta, and Re with respect to the total of Li and themetal elements other than Mo, W, Nb, Ta, and Re on the surface portionsof primary particles is preferably 5 times or more the atomic ratio ofthe whole primary particles.

According to this proposal, the cost reduction, high safety, high loadcharacteristics, and improvement in powder handleability of the lithiumtransition metal compound powder for lithium secondary battery positiveelectrode materials can be achieved all together.

However, the aforementioned lithium transition metal compound powder isobtained by pulverizing a raw material in a liquid medium, spray dryinga slurry in which the pulverized materials are uniformly dispersed, andfiring the obtained spray-dried material. Therefore, some of differentelements such as Mo, W, Nb, Ta, and Re are substituted with Ni disposedin layers, resulting in a reduction in battery characteristics such asbattery capacity and cycle characteristics, which has been a problem.

Further, Japanese Patent Laid-Open No. 2005-251716 proposes a positiveelectrode active material for nonaqueous electrolyte secondary batterieshaving at least a lithium transition metal composite oxide with alayered structure, wherein the lithium transition metal composite oxideis present in the form of particles composed of either or both ofprimary particles and secondary particles as aggregates of the primaryparticles, and wherein the particles have a compound including at leastone selected from the group consisting of molybdenum, vanadium,tungsten, boron, and fluorine at least on the surface.

With that, it is claimed that the positive electrode active material fornonaqueous electrolyte secondary batteries having excellent batterycharacteristics even in more severe use environment is obtained, andthat the initial characteristics are improved without impairing theimprovement in thermostability, load characteristics, and outputcharacteristics particularly by having the compound including at leastone selected from the group consisting of molybdenum, vanadium,tungsten, boron, and fluorine on the surface of the particles.

However, the effect by adding the at least one element selected from thegroup consisting of molybdenum, vanadium, tungsten, boron, and fluorineis to improve the initial characteristics, that is, the initialdischarge capacity and the initial efficiency, where the outputcharacteristics are not mentioned. Further, according to the disclosedproduction method, the firing is performed while the additive element ismixed with a heat-treated hydroxide together with a lithium compound,and therefore the additive element is partially substituted with nickeldisposed in layers to cause a reduction in battery characteristics,which has been a problem.

Further, Japanese Patent Laid-Open No. H11-16566 proposes a positiveelectrode active material in which the circumference of the positiveelectrode active material is coated with a metal containing at least oneselected from Ti, Al, Sn, Bi, Cu, Si, Ga, W, Zr, B, and Mo and/or anintermetallic compound obtained by combining a plurality of theseelements, and/or an oxide.

It is claimed that such coating can ensure the safety by absorbingoxygen gas, but there is no disclosure on the output characteristics.Further, the disclosed production method involves coating using aplanetary ball mill, and such a coating method causes physical damage onthe positive electrode active material, resulting in a reduction inbattery characteristics.

Further, Japanese Patent Laid-Open No. 2010-40383 proposes a positiveelectrode active material heat-treated while a tungstate compound isdeposited on composite oxide particles mainly composed of lithiumnickelate and having a carbonate ion content of 0.15 weight % or less.

According to this proposal, since the tungstate compound or adecomposition product of the tungstate compound is present on thesurface of the positive electrode active material, and the oxidationactivity on the surface of the composite oxide particles during chargeis suppressed, gas generation due to the decomposition of the nonaqueouselectrolyte or the like can be suppressed, but there is no disclosure onthe output characteristics.

Further, the disclosed production method is to deposit a solution inwhich a sulfuric acid compound, a nitric acid compound, a boric acidcompound, or a phosphate compound serving as a deposition component isdissolved in a solvent together with the tungstate compound, on thecomposite oxide particles that are preferably heated to at least theboiling point of the solution in which the deposition component isdissolved, where the solvent is removed within a short time, andtherefore the tungsten compound is not sufficiently dispersed on thesurface of the composite oxide particles and is not uniformly deposited,which has been a problem.

Further, improvements in power enhancement by lithium nickel compositeoxide have been made.

For example, Japanese Patent Laid-Open No. 2013-125732 proposes apositive electrode active material for nonaqueous electrolyte secondarybatteries having fine particles containing lithium tungstate representedby any one of Li2WO4, Li4WO5, and Li6W2O9 on the surface of alithium-metal composite oxide composed of primary particles andsecondary particles formed by aggregation of the primary particles,where high power is supposed to be obtained together with high capacity.

There are, however, more highly demanded power enhancement and capacityenhancement, and further enhancements are demanded. In addition, thereis the following problem: a step of dispersing tungsten in lithium-metalcomposite oxide and a heat-treating step are required for forming fineparticles containing lithium tungstate, thereby causing a reduction inproductivity.

In view of such problems, it is an object of the present invention toprovide a positive electrode active material for nonaqueous electrolytesecondary batteries, at high productivity, which allows further improvedbattery capacity and output characteristics to be obtained when used asa positive electrode of a battery.

SUMMARY

As a result of diligent studies on the powder characteristics oflithium-metal composite oxide used as a positive electrode activematerial for nonaqueous electrolyte secondary batteries and the effectthereof on the positive electrode resistance of the battery, for solvingthe aforementioned problems, the inventors have found that a compoundcontaining tungsten and lithium can be formed on the surface of primaryparticles capable of contacting with an electrolyte, by adding andstirring a poorly soluble tungsten compound in water washing of alithium-metal composite oxide powder, followed by solid-liquidseparation and heat treatment. Further, they have found that a compoundcontaining tungsten and lithium can be formed on the surface of primaryparticles to thereby reduce the positive electrode resistance of abattery and improve output characteristics, thereby accomplishing thepresent invention.

More specifically, the first aspect of the present invention is a methodfor producing a positive electrode active material for nonaqueouselectrolyte secondary batteries, including: a mixing step of adding atungsten compound powder having a solubility A adjusted to 2.0 g/L orless to a lithium-metal composite oxide powder and stirring them inwater washing, the solubility A being determined by stirring thetungsten compound in water having a pH of 12.5 at 25° C. for 20 minutes,the lithium-metal composite oxide powder being represented by thegeneral formula: LicNi1-x-yCoxMyO2 (where 0≤x≤0.35, 0≤y≤0.35, and0.97≤c≤1.25 are satisfied, and M is at least one element selected fromMn, V, Mg, Mo, Nb, Ti and Al) and composed of primary particles andsecondary particles formed by aggregation of the primary particles,followed by solid-liquid separation, to thereby obtain atungsten-containing mixture with the tungsten compound dispersed in thelithium-metal composite oxide powder; and a heat-treating step ofheat-treating the tungsten-containing mixture obtained in the mixingstep and thus uniformly dispersing tungsten on the surface of theprimary particles of the lithium-metal composite oxide powder, andthereafter forming a compound containing tungsten and lithium fromtungsten dispersed uniformly on the surface of the primary particles andlithium in the tungsten-containing mixture, on the surface of theprimary particles of the lithium-metal composite oxide powder.

The second aspect of the present invention is the method for producing apositive electrode active material for nonaqueous electrolyte secondarybatteries according to the first aspect, wherein the tungsten compoundadded in the mixing step is lithium tungstate.

The third aspect of the present invention is the method for producing apositive electrode active material for nonaqueous electrolyte secondarybatteries according to the first and second aspects, wherein the heattreatment in the heat-treating step is performed at a temperature of 100to 600° C. in an oxygen atmosphere or in a vacuum atmosphere.

The fourth aspect of the present invention is the method for producing apositive electrode active material for nonaqueous electrolyte secondarybatteries according to the first to third aspects, wherein the amount oftungsten contained in the tungsten compound is 0.05 to 3.0 at % withrespect to the total number of atoms of Ni, Co and M contained in thelithium-metal composite oxide powder mixed.

The fifth aspect of the present invention is the method for producing apositive electrode active material for nonaqueous electrolyte secondarybatteries according to the first to fourth aspects, wherein the tungstencompound added in the mixing step includes 80% or more of(Li2WO4)7(H2O)4.

The sixth aspect of the present invention is the method for producing apositive electrode active material for nonaqueous electrolyte secondarybatteries according to the second aspect, wherein the lithium tungstateis obtained by a reaction of lithium hydroxide and a tungsten compound.

The seventh aspect of the present invention is the method for producinga positive electrode active material for nonaqueous electrolytesecondary batteries according to the first to sixth aspects, wherein thetemperature in the water washing in the mixing step is 40° C. or less.

The eighth aspect of the present invention is a positive electrodeactive material for nonaqueous electrolyte secondary batteries,including a lithium-metal composite oxide represented by the generalformula (1): LiaNi1-x-yCoxMyWzO2+α (where 0≤x≤0.35, 0≤y≤0.35,0.0005≤z≤0.030, 0.97≤a≤1.25, and 0≤α≤0.20 are satisfied, and M is atleast one element selected from Mn, V, Mg, Mo, Nb, Ti and Al) andcomposed of primary particles and secondary particles formed byaggregation of the primary particles, wherein a compound containinglithium and tungsten is formed on the surface of the primary particlesof the lithium-metal composite oxide, and when voids between the primaryparticles at any 20 or more points are EDX analyzed in transmission-typeelectron microscope observation of a cross section of the secondaryparticles, tungsten is detected at 50% or more of the number of thevoids analyzed.

The ninth aspect of the present invention is the positive electrodeactive material for nonaqueous electrolyte secondary batteries accordingto the eighth aspect, wherein the compound containing lithium andtungsten is present on the surface of the primary particles of thelithium-metal composite oxide as coating thin film having a filmthickness of 1 to 100 nm.

The tenth aspect of the present invention is the positive electrodeactive material for nonaqueous electrolyte secondary batteries accordingto the eighth and ninth aspects, wherein the compound containing lithiumand tungsten is present on the surface of the primary particles of thelithium-metal composite oxide in both forms of thin film having a filmthickness of 1 to 100 nm and fine particles having a particle size of 1to 200 nm.

The eleventh aspect of the present invention is the positive electrodeactive material for nonaqueous electrolyte secondary batteries accordingto the eighth to tenth aspects, wherein the amount of tungsten containedin the compound containing lithium and tungsten is 0.08 to 3.0 at % interms of the number of W atoms with respect to the total number of atomsof Ni, Co and M contained in the lithium-metal composite oxide.

The twelfth aspect of the present invention is the positive electrodeactive material for nonaqueous electrolyte secondary batteries accordingto the eighth to eleventh aspects, wherein the lithium-metal compositeoxide is represented by the general formula (2): LibNi1-x-yCoxMyO2(where 0≤x≤0.35, 0≤y≤0.35, and 0.95≤b≤1.20 are satisfied, and M is atleast one element selected from Mn, V, Mg, Mo, Nb, Ti and Al).

According to the present invention, a positive electrode active materialfor nonaqueous electrolyte secondary batteries, which is capable ofachieving high power together with further improved high capacity whenused as a positive electrode of a battery.

Further, the production method thereof is easy and can allow forproduction on an industrial scale at high productivity, and theindustrial value thereof is exceptionally large.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic illustration of an equivalent circuit used formeasurement examples of impedance evaluation and analysis.

FIG. 2 is an image at 25000-fold magnification by transmission-typeelectron microscope observation of a cross section of the positiveelectrode active material of the present invention.

FIG. 3 is a mapping diagram of results of analysis with accessory EDX oftungsten in the field of view of FIG. 2.

FIG. 4 is a schematic sectional view of a 2032-type coin battery 1 usedfor battery evaluation.

DETAILED DESCRIPTION

Hereinafter, the present invention will be described. A positiveelectrode active material of the present invention will be firstdescribed, and thereafter a production method thereof will be described.

(1) Positive Electrode Active Material

The positive electrode active material for nonaqueous electrolytesecondary batteries obtained by the present invention is a positiveelectrode active material for nonaqueous electrolyte secondarybatteries, including a lithium-metal composite oxide represented by thegeneral formula (1): Li_(a)Ni_(1-x-y)Co_(x)M_(y)W_(z)O_(2+α)(where0≤x≤0.35, 0≤y≤0.35, 0.0005≤z≤0.030, 0.97≤a≤1.25, and 0≤α0.20 aresatisfied, and M is at least one element selected from Mn, V, Mg, Mo,Nb, Ti and Al) and composed of primary particles and secondary particlesformed by aggregation of the primary particles, wherein a compoundcontaining lithium and tungsten is formed on the surface of the primaryparticles of the lithium-metal composite oxide, and when voids betweenthe primary particles forming secondary particles at any 20 or morepoints are EDX analyzed in transmission-type electron microscopeobservation of a cross section of the secondary particles, tungsten isdetected at 50% or more of the number of the voids analyzed.

In the present invention, high charge-discharge capacity is obtained byusing the lithium-metal composite oxide represented by the generalformula (2) Li_(b)Ni_(1-x-y)Co_(x)M_(y)O₂ (where 0≤x≤0.35, 0≤y≤0.35, and0.95≤b≤1.20 are satisfied, and M is at least one element selected fromMn, V, Mg, Mo, Nb, Ti, and Al) as a base material.

Further, the base material is in the form of a lithium-metal compositeoxide composed of primary particles and secondary particles formed byaggregation of the primary particles (hereinafter, the secondaryparticles and the primary particles existing alone may be referred tocollectively as “lithium-metal composite oxide particles”), and acompound containing lithium and tungsten formed on the surface of theprimary particles (primary particle surface) allows battery performancessuch as battery capacity and output characteristics to be achieved athigher level.

Generally, when the surface of the positive electrode active material ishere completely coated with a different compound, the movement(intercalation) of lithium ions is significantly limited, and thereforehigh capacity that is an advantage of lithium nickel composite oxide iseventually offset.

In contrast, in the positive electrode active material for nonaqueouselectrolyte secondary batteries of the present invention (which will behereinafter referred to simply as “positive electrode active material”),a compound containing lithium (Li) and tungsten (W) (which may behereinafter referred to simply as “compound”) is formed on the surfaceof the lithium-metal composite oxide particles, and the compound hashigh lithium ion conductivity and has an effect of promoting themovement of lithium ions. Therefore, the abovementioned compound isformed on the surface of the lithium-metal composite oxide particles,thereby forming Li conduction paths at the interface with theelectrolyte, so that the reaction resistance of the positive electrodeactive material (which may be hereinafter referred to as “positiveelectrode resistance”) is reduced to improve output characteristics.

More specifically, the reduction in positive electrode resistancereduces the voltage to be lost in the battery, and the voltage actuallyapplied to the load side is relatively increased, thereby allowing highpower to be obtained. Further, the increase in the voltage applied tothe load side allows lithium to be sufficiently inserted into andremoved from the positive electrode, and therefore battery capacity isalso improved. Further, the reduction in reaction resistance can alsoreduce the load of the active material during charge-discharge tothereby improve cycle characteristics.

The compound contains Li and W, to thereby impart high Li ionconductivity and the effect of promoting the movement of Li ions, and50% or more of W contained in the compound is preferably present in theform of Li4WO5.

More specifically, Li4WO5 has many Li ion conduction paths and has ahigh effect of promoting the movement of Li ions, as the compoundcontaining Li and W, and therefore 50% or more of W is present in theform of Li4WO5, thereby allowing a higher effect of reducing thereaction resistance to be obtained.

The contact with the electrolyte occurs on the surface of the primaryparticles, and therefore it is important that the compound be formed onthe surface of the primary particles.

Here, the surface of the primary particles in the present inventionincludes the surface of the primary particles exposed on the outersurface of the secondary particles, and the surface of the primaryparticles communicating with the outside of the secondary particles soas to allow the electrolyte to penetrate therethrough and exposed intovoids in the vicinity of the surface of the secondary particles andinside thereof. Further, the surface of the primary particles in thepresent invention includes even the grain boundaries between the primaryparticles if the primary particles are not perfectly bonded and theelectrolyte can penetrate therethrough.

More specifically, the contact of the compound with the electrolyteoccurs not only on the outer surface of the secondary particles formedby aggregation of the primary particles, but also in the voids betweenthe primary particles in the vicinity of the surface of the secondaryparticles and inside thereof and further at the imperfect grainboundaries between the primary particles forming the secondaryparticles, and therefore it is necessary to form the compound also onthe surface of the primary particles to promote the movement of lithiumions.

Accordingly, the reaction resistance of the lithium-metal compositeoxide particles can be further reduced by forming the compound more onthe surface of the primary particles which can contact with theelectrolyte.

In the positive electrode active material of the present invention, whenvoids between the primary particles at any 20 or more points present inthe secondary particles, namely, the surface of the primary particlesfacing the voids are EDX analyzed in transmission-type electronmicroscope observation of a cross section of the secondary particles,tungsten is detected at 50% or more of the number of the voids analyzed,preferably 60% or more, more preferably 70% or more.

The tungsten on the surface of the primary particles is taken togetherwith Li to easily form a compound, and it can be thus said that tungstendetection exhibits formation of a compound containing Li and W.Therefore, the compound is formed at sufficient positions on the surfaceof the primary particles which can contact with the electrolyte, therebyenabling higher battery performances to be achieved. If the number ofvoids where tungsten is detected is less than 50% with respect to thenumber of the voids analyzed, the surface of the primary particles, onwhich the compound is not formed, is increased, and therefore batteryperformances are not sufficiently enhanced.

Further, as the form of the compound on the surface of the primaryparticles, when the surface of the primary particles is coated withlayered materials, the contact area with the electrolyte is reduced, andwhen such layered materials are formed, the formation of the compoundtends to concentrate on the surface of some specific primary particles.Accordingly, since the layered materials as coating materials have highlithium ion conductivity, the effects of improving the charge-dischargecapacity and reducing the reaction resistance are obtained, but they arenot sufficient, leaving room for improvement

Meanwhile, the surface of the primary particles is coated with a thinfilm, thereby enabling Li conduction paths to be formed at the interfacewith the electrolyte while the reduction in specific surface area issuppressed, and higher effects of improving the charge-dischargecapacity and reducing the reaction resistance are obtained.

In the case where the surface of the primary particles is coated withsuch a compound in the form of thin films, the compound is preferablypresent on the surface of the primary particles of the lithium-metalcomposite oxide as thin films having a film thickness of 1 to 100 nm.

When the film thickness is less than 1 nm, the coating films may fail tohave sufficient lithium ion conductivity in some cases. Further, whenthe film thickness is over 100 nm, the coating films are in the state ofthe layered materials, the contact area with the electrolyte is reduced,a higher effect of reducing the reaction resistance is not obtained insome cases, and also thin film formation on the surface of the primaryparticles is non-uniform in some cases.

Further, also in the case where the compound is formed on the surface ofthe primary particles in the form of fine particles as well as in theform of a coating thin film, a high effect on battery characteristics isobtained.

The compound in the form of fine particles is preferably present on thesurface of the primary particles of the lithium-metal composite oxide,as fine particles having a particle size of 1 to 200 nm, for obtaining ahigher effect of improving the battery characteristics.

The contact area with the electrolyte is rendered sufficient by havingsuch a form, so that the lithium ion conduction can be effectivelyimproved, thereby allowing the reaction resistance to be moreeffectively reduced and the charge-discharge capacity to be improved.When the particle size is less than 1 nm, the fine particles may fail tohave sufficient lithium ion conductivity in some cases.

When the particle size is over 200 nm, however, the formation of thefine particles on the surface of the primary particles is madenon-uniform, which may result in failure to obtain a higher effect ofreducing the reaction resistance in some cases.

Not all which are present in the form of fine particles, however, arenecessarily present as fine particles having a particle size of 1 to 200nm, and a higher effect of improving the battery characteristics isobtained by interaction with a thin coating film when 50% or more of thenumber of the fine particles formed on the surface of the primaryparticles are preferably formed to have a particle size in the range of1 to 200 nm.

Meanwhile, in the case where thin films and fine particles arenon-uniformly formed between the secondary particles of thelithium-metal composite oxide, the movement of lithium ions between thesecondary particles is rendered non-uniform, and therefore a load isapplied onto some specific secondary particles, which tends to cause adeterioration in cycle characteristics and an increase in reactionresistance.

Accordingly, thin films and fine particles are preferably uniformlyformed also between the secondary particles.

The amount of the compound formed can be controlled by the amount oftungsten contained in the compound, and tungsten is contained in thecompound in an amount of 0.05 to 3.0 at % as the number of W atoms withrespect to the total number of atoms of Ni, Co and M contained in thepositive electrode active material, preferably 0.08 to 3.0 at %, morepreferably 0.1 to 1.50 at %, further preferably 0.15 to 1.00 at %.

Thus, while the surface of the primary particles which can contact withthe electrolyte is ensured, the compound can be formed in a sufficientamount and the reaction resistance of the positive electrode activematerial can be reduced. Further, even the inside of the secondaryparticles can efficiently contribute to charge-discharge, and thereforecan also improve the battery capacity.

When tungsten is contained in the compound in an amount of less than0.05 at % as the number of W atoms with respect to the total number ofatoms of Ni, Co and M contained in the positive electrode activematerial, the amount of the compound formed is small and the reactionresistance of the positive electrode active material cannot besufficiently reduced in some cases. Meanwhile, when tungsten iscontained in an amount of more than 3.00 at %, the surface of theprimary particles which can contact with the electrolyte is decreasedand the charge-discharge efficiency inside the secondary particles isreduced, thereby reducing the effect of improving the battery capacityin some cases.

Next, the void content measured in cross-sectional observation of thesecondary particles of the positive electrode active material of thepresent invention is preferably 0.15 to 3%, more preferably 0.15 to1.5%, further preferably 0.15 to 0.5%. The voids are present in such avoid content, thereby allowing the electrolyte to penetrate into thesurface of the secondary particles, and enabling the surface of theprimary particles which can contact with the electrolyte to besufficiently ensured.

The surface of the primary particles which can contact with theelectrolyte can also be controlled by the specific surface area, and thespecific surface area of the positive electrode active material,measured by the BET method, is preferably 0.9 to 1.5 m2/g. Thus, whilethe surface of the primary particles which can contact with theelectrolyte is controlled so as to have a proper area, thereby allowingthe safety of the positive electrode active material to be ensured, highbattery characteristics can be obtained.

Here, the void content can be measured by observing any cross section ofthe secondary particles with a scanning electron microscope (SEM), andperforming image analysis.

For example, the void content can be determined as follows: a pluralityof the secondary particles are embedded into a resin or the like,cross-section polishing or the like is performed thereon to enable thecross section of the particles to be observed, thereafter image analysissoftware: WinRoof 6.1.1 or the like; is used to adopt the secondaryparticles at any 20 or more points for the parameter and to define thevoid region in the secondary particles as a black region and the compactregion in the profile of the secondary particles as a white region, thetotal of the regions is subjected to measurement to calculate the totalarea of the total number of the secondary particles, and the area ratio[black region area/(black region area+white region area)] is defined asthe void content to calculate the void content.

The positive electrode active material of the present invention hasimproved battery characteristics by forming the compound containing Wand Li on the surface of the primary particles constituting thelithium-metal composite oxide as a base material, to reduce the reactionresistance, and the powder characteristics as the positive electrodeactive material, such as particle size and tap density, may fall withinthe range of those of positive electrode active materials commonly used.

Further, the Li content in the compound containing Li and W moreincreases than the lithium content in the lithium-metal composite oxide,in the entire positive electrode active material.

Accordingly, with respect to the amount of lithium in the entirepositive electrode active material, the ratio “Li/Me1” of the number ofatoms of Li with respect to the sum (Me1) of the number of atoms of Ni,Co, and M in the positive electrode active material is 0.97 to 1.25,preferably 0.97 to 1.20. When the ratio Li/Me1 is less than 0.97, thereaction resistance of the positive electrode in the nonaqueouselectrolyte secondary battery using the obtained positive electrodeactive material increases, and thus the output of the battery decreases.Further, when Li/Me1 is over 1.25, the discharge capacity of thepositive electrode active material decreases and the reaction resistanceof the positive electrode increases as well. The ratio “Li/Me1” is morepreferably 0.97 to 1.20 for decreasing the reaction resistance and alsoincreasing the discharge capacity to further improve batterycharacteristics.

Further, the ratio “Li/Me2” of the number of atoms of Li with respect tothe sum (Me2) of the number of atoms of Ni, Co and M in thelithium-metal composite oxide is preferably 0.95 to 1.20, morepreferably 0.97 to 1.15.

The ratio “Li/Me2” is set to 0.95 to 1.20, thereby enabling the ratio“Li/Me1” to be easily controlled within the range from 0.97 to 1.25.Further, the lithium-metal composite oxide is excellent in batterycharacteristics such as battery capacity and output characteristics.

The effect by attachment of the compound containing W and Li onto thesurface of the primary particles of the lithium-metal composite oxidecan be applied to, for example, lithium-cobalt composite oxide,lithium-manganese composite oxide and lithium-nickel-cobalt-manganesecomposite oxide powders, and further not only the positive electrodeactive material of the present invention, but also a positive electrodeactive material for lithium secondary batteries which is commonly used.

(2) Method for Producing Positive Electrode Active Material

Hereinafter, a method for producing the positive electrode activematerial for nonaqueous electrolyte secondary batteries of the presentinvention (which will be hereinafter referred to simply as “productionmethod”) will be described in detail for each step.

[Mixing Step]

The mixing step is a step of obtaining a tungsten-containing mixture(which will be hereinafter referred to simply as “mixture”) formed byusing, as a base material, a lithium-metal composite oxide representedby the general formula: LicNi1-x-yCoxMyO2 (where 0≤x≤0.35, 0≤y≤0.35, and0.97≤c≤1.25 are satisfied, and M is at least one element selected fromMn, V, Mg, Mo, Nb, Ti and Al) and composed of primary particles andsecondary particles formed by aggregation of the primary particles,adding and stirring a tungsten compound powder having a solubility of2.0 g/L or less as determined by stirring the tungsten compound in waterhaving a pH of 12.5 at 25° C. for 20 minutes, in water washing a powderof the lithium-metal composite oxide, followed by solid-liquidseparation, to thereby disperse a tungsten compound in the lithium-metalcomposite oxide powder. The lithium-metal composite oxide powder in theform of slurry in water washing (which may be hereinafter referred tosimply as “slurry”) and the tungsten compound powder can be stirred tothereby allow W to be uniformly dispersed in the lithium-metal compositeoxide powder.

Meanwhile, in the case where the solid or solution of the tungstencompound is mixed with the lithium-metal composite oxide powder in theform of powder, the lithium-metal composite oxide in the form of powderless flows and has more difficulty in providing uniform dispersion of Wthan that in the form of slurry. Further, if mixing is performed for along time for uniform dispersion, productivity is significantly reduced.Further, in the case where those in the form of solid (powder) low inwater content are mixed, the tungsten compound insufficiently penetratesinto the secondary particles of the lithium-metal composite oxide, andthe power enhancement effect due to the formation of the compound alsoon the surface of the primary particles in the secondary particles isreduced.

While uniform dispersion can also be achieved by a method where thelithium-metal composite oxide powder is immersed in the solution of thetungsten compound to perform solid-liquid separation, tungsten dissolvedduring solid-liquid separation is lost as a waste liquid, or anyfacilities for large-scale liquid circulation are needed in the case ofrecover or reuse of such tungsten, thereby resulting in an increase inproduction cost.

In the mixing step, when the lithium-metal composite oxide powder iswashed with water, the lithium-metal composite oxide powder in the formof slurry and the tungsten compound powder may be mixed by stirring.Accordingly, water and the lithium-metal composite oxide powder may bemixed to form a slurry and thereafter the tungsten compound powder maybe added into and mixed with the slurry, or the lithium-metal compositeoxide powder may be added to water to which the tungsten compound powderis added in advance, and thereafter stirred and mixed in the form ofslurry. The stirring method may be a common stirring method, andstirring is preferably made at a speed that allows any powder not to beprecipitated such that the powder is uniformly dispersed.

In the production method of the present invention, a tungsten compoundpowder having a solubility A of 2.0 g/L or less is used when thesolubility A is determined by stirring for 20 minutes in water having apH of 12.5 at a water temperature of 25° C. in water washing. Thisallows the stirring in water washing to uniformly mix the lithium-metalcomposite oxide powder and the tungsten compound powder. When thesolubility A is over 2.0 g/L, tungsten dissolved into the liquidcomponent of the slurry increases, and there is caused the same problemas that of the above method where the lithium-metal composite oxidepowder is immersed in the solution of the tungsten compound to performsolid-liquid separation.

Accordingly, for obtaining the effect of reducing the reactionresistance by the formation of the compound on the surface of theprimary particles of the lithium-metal composite oxide powder, a largeamount of the tungsten compound powder is needed to be added, therebyresulting in a significant increase in cost.

The solubility A is defined as the solubility in stirring in waterhaving a pH of 12.5 at a water temperature of 25° C. for 20 minutes. Forexample, 1 g of the tungsten compound powder is added to 100 ml of waterhaving a pH of 12.5 at 25° C., and then stirred for 20 minutes. Afterthe stirring, the amount of the tungsten compound powder dissolved canbe calculated from the analysis value of the tungsten content in water,thereby determining the solubility.

The water temperature and the pH value here are preferably controlledwithin a variation of ±0.5° C. from 25° C. as the center value for watertemperature and within a variation of ±0.2 from 12.5 as the center valuefor pH value to produce no variation in solubility measurement. The pHvalue is here preferably controlled by adjustment with sodium hydroxide,potassium hydroxide, lithium hydroxide, or the like. The slurry in themixing step has a higher pH value as a result of dissolution of lithiumfrom the lithium-metal composite oxide powder, and therefore lithiumhydroxide is more preferably used in control of the pH value at thesolubility A.

The tungsten compound powder may be any tungsten compound powder as longas it has a solubility A of 2.0 g/L or less, but, as the tungstencompound, tungsten oxide, lithium tungstate, ammonium tungstate, sodiumtungstate, and the like are preferable, tungsten oxide, lithiumtungstate, and ammonium tungstate with a low possibility ofcontamination are more preferable, and tungsten oxide and lithiumtungstate are further preferable. In particular, lithium tungstate ispreferable for ensuring the amount of lithium in the compound to enhancethe lithium ion conductivity of the compound, and a tungsten compoundpowder including 80% or more of (Li2WO4)7(H2O)4 is more preferably usedin terms of solubility.

The production method of the lithium tungstate is not specificallylimited, and a method where lithium hydroxide and the tungsten compoundare reacted to provide the lithium tungstate is easy and preferable.

The solubility A is affected also by the particle size of the tungstencompound powder. More specifically, a tungsten compound powder having asmall particle size tends to be increased in the solubility A, and atungsten compound powder having a large particle size tends to bedecreased in the solubility A.

Accordingly, the particle size is preferably adjusted such that thesolubility A is 2.0 g/L or less, and the particle size is morepreferably adjusted such that the solubility A is 1.5 g/L or less.

In the case where the tungsten compound powder has secondary particlesformed by aggregation of primary particles, and aggregated particlesformed by aggregation of the secondary particles, the particle size ofthe primary particles has a large effect on the solubility and thereforethe particle size of the primary particles is more preferably adjusted.

More specifically, the secondary particles and the aggregate arepartially dissolved in water washing, and thus dispersed in a state ofbeing close to the primary particles.

The lower limit of the solubility A may be 0 g/L, namely, even atungsten compound powder not dissolved at all can also be used becauseof being dissolved during heat treatment as described below, but thesolubility A is preferably 0.1 g/L or more, more preferably 0.5 g/L ormore for allowing tungsten to be dispersed into the secondary particles.

Further, the solubility B in water washing, at a water temperatureincreased from 25° C. to 50° C., is preferably over 2.0 g/L, morepreferably over 3.0 g/L from the viewpoint that the tungsten compound issufficiently dissolved due to the temperature rise during heat treatmentand is allowed to penetrate into the surface of the primary particlesinside the secondary particles.

Such a tungsten compound increased in solubility due to the temperaturerise can be used to thereby allow tungsten to be uniformly dispersed inthe lithium-metal composite oxide powder.

The average particle size as the particle size of the primary particlesin the tungsten compound powder is preferably 0.2 to 5 μm, preferably0.3 to 3 μm.

When the average particle size is less than 0.2 μm, the solubility A isover 2.0 g/L in some cases. Meanwhile, when the average particle size isover 5 μm, the tungsten compound powder, even if macroscopicallyuniformly dispersed in the mixture obtained by solid-liquid separationafter water washing, is microscopically non-uniformly dispersed in termsof particles forming the lithium-metal composite oxide in some cases.Further, when the average particle size is over 5 μm, partialdissolution in water washing is also decreased.

The amount of tungsten contained in the mixture corresponds to theamount of tungsten in the positive electrode active material, andtherefore is preferably adjusted to 0.05 to 3.0 at % with respect to thetotal number (Me) of atoms of nickel, cobalt and M contained in thelithium-metal composite oxide mixed.

Further, in the production method of the present invention, solid-liquidseparation is performed after water washing, and therefore tungstendissolved in the slurry, excluding the liquid component contained in themixture, is discharged. The amount of tungsten dissolved in the slurrycorresponds to the amount which is stable depending on the slurrytemperature, stirring conditions, and the tungsten compound powder used,and therefore the amount of tungsten in the mixture can be easilyadjusted within the above range by determining in advance the amount oftungsten dissolved and the water content after solid-liquid separationby a preliminary test or the like.

The temperature for water washing, namely, the slurry temperature ispreferably set to 40° C. or less, more preferably 35° C. or less.

Since the tungsten compound powder is also increased in solubility inaccordance with an increase in slurry temperature, the slurrytemperature can be set to 40° C. or less, thereby inhibiting thetungsten compound powder from being excessively dissolved in the slurry.

The lower limit of the slurry temperature is not specifically limited,and may be a temperature at which the effect of removing excess lithiumpresent on the surface of the lithium-metal composite oxide powder bywashing is obtained, and the slurry temperature is preferably set to 10°C. or more, more preferably 15° C. or more.

The slurry concentration in water washing, in terms of the solid-liquidratio, namely, the amount of the lithium-metal composite oxide powderwith respect to 1 L of water, is preferably set to 200 to 5000 g, morepreferably 500 to 1500 g.

The solid-liquid ratio falls within the above range, thereby inhibitinglithium from being excessively dissolved from the lithium-metalcomposite oxide powder and also allowing the pH value of the slurry tofall within a proper range, and thus the tungsten compound powder can beinhibited from being excessively dissolved. Further, the pH value of theslurry is preferably set to 11.5 to 13.5.

In the production method of the present invention, the lithium-metalcomposite oxide powder as a base material is washed with water and thuslithium is dissolved in the slurry in water washing. Accordingly, theamount decreased from Li/Me before water washing may be confirmed by apreliminary test to determine the Li/Me2 after water washing (whichcorresponds to b in the general formula (2)) in advance, and alithium-metal composite oxide powder whose Li/Me is adjusted may be usedas a material before water washing.

The amount of the Li/Me to be decreased under common water washingconditions is about 0.03 to 0.08.

Accordingly, as the lithium-metal composite oxide before water washing,a known lithium-metal composite oxide represented by the generalformula: LicNi1-x-yCoxMyO2 (where 0≤x≤0.35, 0≤y≤0.35, and 0.97≤c≤1.25are satisfied, and M is at least one element selected from Mn, V, Mg,Mo, Nb, Ti and Al) is used in terms of high capacity and low reactionresistance.

Further, since it is advantageous to increase the contact area with theelectrolyte for improving the output characteristics, it is preferableto use a lithium-metal composite oxide powder composed of primaryparticles and secondary particles formed by aggregation of the primaryparticles and to have voids and grain boundaries through which theelectrolyte can penetrate in the secondary particles.

[Heat-Treating Step]

The heat-treating step is a step of heat-treating the mixture obtainedin the mixing step, thereby allowing tungsten (W) to be uniformlydispersed on the surface of the primary particles of the lithium-metalcomposite oxide powder, and further forming the compound containingtungsten and lithium from the tungsten and lithium (Li) in the mixture,on the surface of the primary particles of the lithium-metal compositeoxide.

Thus, the compound containing W and Li is formed from W supplied fromthe tungsten compound in the mixture, Li in the mixture, and also Li inthe tungsten compound in the case of use of a tungsten compoundcontaining Li, thereby providing a positive electrode active materialfor nonaqueous electrolyte secondary batteries including the compoundcontaining W and Li on the surface of the primary particles of thelithium-metal composite oxide.

The lithium-metal composite oxide powder is washed with water to therebyallow excess lithium to be dissolved. Therefore, water contained in themixture includes lithium dissolved in water washing.

Further, when the compound containing W and Li is formed, the compoundis formed also by a reaction with excess lithium not dissolved and Li inthe crystal of a part of the lithium-metal composite oxide.

Accordingly, Li in the mixture includes lithium dissolved in waterwashing and contained in water in the mixture, excess lithium notdissolved, and Li in the crystal of the lithium-metal composite oxide.

Any conditions may be adopted as the heat treatment conditions as longas such conditions allow the mixture to be dried and also the compoundcontaining W and Li to be formed, but the heat treatment is preferablyperformed at a temperature of 100 to 600° C. in an oxygen atmosphere ora vacuum atmosphere for preventing the deterioration in electricalproperties in use as a positive electrode active material for nonaqueouselectrolyte secondary batteries. Further, the tungsten compound powderin the mixture is dissolved in water contained in the mixture by heatingin the heat treatment, and thus W can be uniformly dispersed in thesurface of the primary particles of the lithium-metal composite oxide,namely, not only the surface of the secondary particles, but also voidsin the vicinity of the surface of the secondary particles and inside thesecondary particles, and also the imperfect grain boundaries.

When the heat treatment temperature is less than 100° C., water is notsufficiently evaporated, which may result in failure to sufficientlyform the compound. Further, the tungsten compound powder may also beinsufficiently dissolved to reduce the penetration of W inside thesecondary particles.

Meanwhile, when the heat treatment temperature exceeds 600° C., theprimary particles of the lithium-metal composite oxide are fired, and Wpartially forms a solid solution in the layered structure of thelithium-metal composite oxide, which may reduce the charge-dischargecapacity of the battery.

Further, the rate of temperature increase is preferably set to 0.8 to1.2° C./minute, until the tungsten compound contained in the mixture issufficiently dissolved, for example, until the temperature exceeds 90°C.

Setting the rate of temperature increase as above allows the tungstencompound to be sufficiently dissolved during the temperature rise so asto penetrate into the surface of the primary particles inside thesecondary particles.

The atmosphere in the heat treatment is preferably an atmospheresubjected to decarburization treatment, preferably an oxidizingatmosphere such as an oxygen atmosphere or a vacuum atmosphere, foravoiding a reaction with water and carbonic acid in the atmosphere.

The heat treatment time is not specifically limited but is preferably 5to 15 hours for sufficiently evaporating the water to form the compoundwhile allowing W to penetrate inside the secondary particles in themixture.

(3) Nonaqueous Electrolyte Secondary Battery

The nonaqueous electrolyte secondary battery provided by the presentinvention is constituted by a positive electrode, a negative electrode,a nonaqueous electrolyte, etc., and constituted by the same componentsas those of common nonaqueous electrolyte secondary batteries. Theembodiment described below is just an example, and the nonaqueouselectrolyte secondary battery of the present invention can beimplemented by employing embodiments in which various changes andimprovements are made, using the embodiment shown in this description asa base, based on the knowledge of those skilled in the art. Further, theapplications of the nonaqueous electrolyte secondary battery of thepresent invention are not specifically limited.

(a) Positive Electrode

Using the positive electrode active material for nonaqueous electrolytesecondary batteries described above, the positive electrode of thenonaqueous electrolyte secondary battery is produced, for example, asfollows.

First, a positive electrode active material in powder form, a conductivematerial, and a binder are mixed, and activated carbon and a solvent forits intended purpose such as a viscosity adjuster are further added, asneeded, and the mixture is kneaded to produce a positive electrodecomposite material paste.

The mixing ratio of each component in the positive electrode compositematerial paste is also an important element to determine the performanceof the nonaqueous electrolyte secondary battery. When the total mass ofthe solid contents of the positive electrode composite materialexcluding the solvent is taken as 100 parts by mass, it is desirablethat the content of the positive electrode active material be 60 to 95parts by mass, the content of the conductive material be 1 to 20 partsby mass, and the content of the binder be 1 to 20 parts by mass, as in apositive electrode of a common nonaqueous electrolyte secondary battery.

The obtained positive electrode composite material paste, for example,is applied to the surface of a current collector made of aluminum foil,followed by drying, to disperse the solvent. In order to enhance theelectrode density, it may be pressed by roll pressing or the like, asneeded. Thus, a positive electrode in sheet form can be produced.

The positive electrode in sheet form can be used for producing abattery, for example, by being cut into a suitable size corresponding tothe intended battery. However, the method for producing the positiveelectrode is not limited to the aforementioned example, and anothermethod may be employed.

For producing the positive electrode, graphite (such as naturalgraphite, artificial graphite, and expanded graphite) and carbon blackmaterials such as acetylene black and Ketjen black (R), for example, canbe used as the conductive material.

The binder serves to hold the active material particles, for whichpolyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE),fluororubber, ethylene propylene diene rubber, styrene butadiene,cellulose resins, and polyacrylic acid, for example, can be used.

As needed, the positive electrode active material, the conductivematerial, and the activated carbon are dispersed, and a solvent todissolve the binder is added to the positive electrode compositematerial. Specifically, an organic solvent such asN-methyl-2-pyrrolidone can be used as the solvent. Further, activatedcarbon can be added to the positive electrode composite material for anincrease in electric double layer capacity.

(b) Negative Electrode

As the negative electrode, a material formed by applying a negativeelectrode composite material formed into a paste by mixing the binderwith metal lithium, lithium alloy, or the like, or a negative electrodeactive material capable of absorbing and desorbing lithium ions andadding a suitable solvent onto the surface of the current collector madeof a metal foil such as copper, followed by drying and compressing forincreasing the electrode density, as needed, is used.

As the negative electrode active material, a powder material of naturalgraphite, artificial graphite, a fired material of an organic compoundsuch as a phenolic resin, and a carbon material such as cokes, forexample, can be used.

In this case, a fluorine-containing resin such as PVDF can be used asthe negative electrode binder, as in the positive electrode, and anorganic solvent such as N-methyl-2-pyrrolidone can be used as thesolvent to disperse the active material and the binder therein.

(c) Separator

A separator is interposed between the positive electrode and thenegative electrode. The separator separates the positive electrode andthe negative electrode from each other and holds the electrolyte. A thinfilm of polyethylene, polypropylene, or the like having a large numberof fine holes can be used as the separator.

(d) Non-Aqueous Electrolyte

The nonaqueous electrolyte is formed by dissolving a lithium salt as asupporting salt in an organic solvent.

As the organic solvent used, one selected from cyclic carbonates such asethylene carbonate, propylene carbonate, butylene carbonate, andtrifluoropropylene carbonate, chain carbonates such as diethylcarbonate, dimethyl carbonate, ethyl methyl carbonate, and dipropylcarbonate, ether compounds such as tetrahydrofuran,2-methyltetrahydrofuran, and dimethoxyethane, sulfur compounds such asethyl methyl sulfone and butanesulton, and phosphorus compounds such astriethyl phosphate and trioctyl phosphate can be used alone, or two ormore of these can be mixed for use.

Further, as the supporting salt, LiPF6, LiBF4, LiClO4, LiAsF6, andLiN(CF3S02)2, and composite salts of these can be used.

Further, the non-aqueous electrolyte may contain a radical scavenger, asurfactant, a flame retardant, and the like.

(e) Shape and Configuration of Battery

The nonaqueous electrolyte secondary battery of the present inventionconstituted by the positive electrode, the negative electrode describedabove, the separator, and the non-aqueous electrolyte described abovecan have various shapes such as a cylindrical type and a stacked type.

Even if any shape is employed, an electrode body is obtained by stackingthe positive electrode and the negative electrode via the separator, theobtained electrode body is impregnated with the non-aqueous electrolyte,the connection between the positive electrode current collector and thepositive electrode terminal connected to the outside and the connectionbetween the negative electrode current collector and the negativeelectrode terminal connected to the outside are established using leadsfor the current collectors, and the components are sealed in a batterycase, to complete the nonaqueous electrolyte secondary battery.

(f) Characteristics

The nonaqueous electrolyte secondary battery using the positiveelectrode active material of the present invention has high capacity andhigh power.

In particular, the nonaqueous electrolyte secondary battery obtained bya further preferable embodiment using the positive electrode activematerial according to the present invention, for example, when used as apositive electrode of a 2032-type coin battery, has a high initialdischarge capacity of 165 mAh/g or more and a low positive electroderesistance and further has high capacity and high power. Further, italso has high thermostability and excellent safety.

Here, the method for measuring the positive electrode resistance in thepresent invention is exemplified as follows.

When the frequency dependence of a battery reaction is measured by acommon AC impedance method as an electrochemical evaluation technique, aNyquist diagram based on the solution resistance, the negative electroderesistance and the negative electrode capacity, and the positiveelectrode resistance and the positive electrode capacity is obtained asshown in FIG. 1.

The battery reaction in an electrode is made by the resistancecomponents following charge transfers and the capacity components by anelectric double layer. When these components are shown as an electricalcircuit, a parallel circuit of the resistance and the capacity isobtained, and they are shown as an equivalent circuit in which thesolution resistance and the parallel circuit of the negative electrodeand the positive electrode are connected in series as the entirebattery.

The Nyquist diagram determined is subjected to fitting calculation usingthe equivalent circuit, and the resistance components and the capacitycomponents each can be estimated.

The positive electrode resistance is equal to the diameter of asemicircle on the low frequency side of the Nyquist diagram to beobtained.

From above, the positive electrode resistance can be estimated byperforming the AC impedance measurement on the produced positiveelectrode and subjecting the obtained Nyquist diagram to fittingcalculation using the equivalent circuit.

EXAMPLES

A secondary battery having a positive electrode using the positiveelectrode active material obtained by the present invention wasproduced, and the performance (initial discharge capacity and positiveelectrode resistance) was measured to thereby also evaluate the positiveelectrode active material according to the present invention, incombination.

Hereinafter, the present invention will be specifically described by wayof examples, but the present invention is not limited to these examplesat all.

(Production and Evaluation of Battery)

For evaluating the positive electrode active material, a 2032-type coinbattery 1 (which will be hereinafter referred to as coin type battery)shown in FIG. 4 was used.

As shown in FIG. 4, the coin type battery 1 is constituted by a case 2and electrodes 3 housed in the case 2.

The case 2 has a hollow positive electrode can 2 a with one end open anda negative electrode can 2 b arranged in the opening of the positiveelectrode can 2 a, and is configured so that, when the negativeelectrode can 2 b is arranged in the opening of the positive electrodecan 2 a, a space to house the electrodes 3 is formed between thenegative electrode can 2 b and the positive electrode can 2 a.

The electrodes 3 are constituted by a positive electrode 3 a, aseparator 3 c, and a negative electrode 3 b, which are stacked to bealigned in this order and are housed in the case 2 so that the positiveelectrode 3 a is in contact with the inner surface of the positiveelectrode can 2 a, and the negative electrode 3 b is in contact with theinner surface of the negative electrode can 2 b.

The case 2 includes a gasket 2 c, and the relative movement between thepositive electrode can 2 a and the negative electrode can 2 b is fixedby the gasket 2 c so that the non-contact state is maintained. Further,the gasket 2 c also has a function of sealing the gap between thepositive electrode can 2 a and the negative electrode can 2 b so as toblock between the inside and the outside of the case 2 air-tightly andliquid-tightly.

The coin type battery 1 shown in FIG. 4 was fabricated as follows.

First, 52.5 mg of the positive electrode active material for nonaqueouselectrolyte secondary batteries, 15 mg of acetylene black, and 7.5 mg ofpolytetrafluoroethylene resin (PTFE) were mixed, followed by pressmolding at a pressure of 100 MPa to a diameter of 11 mm and a thicknessof 100 μm, to produce the positive electrode 3 a. The thus producedpositive electrode 3 a was dried in a vacuum dryer at 120° C. for 12hours.

Using the positive electrode 3 a, the negative electrode 3 b, theseparator 3 c, and the electrolyte, the coin type battery 1 describedabove was produced in a glove box under Ar atmosphere with the dew pointcontrolled to −80° C.

As the negative electrode 3 b, a negative electrode sheet in whichgraphite powder with an average particle size of about 20 μm andpolyvinylidene fluoride were applied to a copper foil and which waspunched into a disk shape with a diameter of 14 mm was used.

As the separator 3 c, a polyethylene porous film with a film thicknessof 25 μm was used. As the electrolyte, an equal mixture (manufactured byTOMIYAMA PURE CHEMICAL INDUSTRIES, LTD.) of ethylene carbonate (EC) anddiethyl carbonate (DEC) with 1 M LiClO4 serving as a supportingelectrolyte was used.

The initial discharge capacity and the positive electrode resistanceshowing the performance of the thus produced coin type battery 1 weremeasured and evaluated as follows.

The capacity when the coin type battery 1 allowed to stand for about 24hours from the fabrication was charged, with the current density withrespect to the positive electrode set to 0.1 mA/cm2, to a cut-offvoltage of 4.3 V after the OCV (Open Circuit Voltage) became stable,followed by a pause for one hour, and was discharged to a cut-offvoltage of 3.0 V was taken as the initial discharge capacity.

Further, the Nyquist plot shown in FIG. 1 is obtained by charging thecoin type battery 1 at a charge potential of 4.1 V and measuring thepositive electrode resistance using a frequency response analyzer and apotentio-galvanostat (1255B, manufactured by Solartron) by the ACimpedance method.

Since the Nyquist plot is shown as the sum of characteristic curvesshowing the solution resistance, the negative electrode resistance andthe capacity thereof, and the positive electrode resistance and thecapacity thereof, fitting calculation was performed based on the Nyquistplot using the equivalent circuit to calculate the value of the positiveelectrode resistance.

In the present examples, the positive electrode active material, and thesecondary battery, the respective samples of special reagentsmanufactured by Wako Pure Chemical Industries, Ltd. were used forproducing the composite hydroxide.

Example 1

A powder of lithium-metal composite oxide represented byLi1.03Ni0.82Co0.15Al0.03O2 and obtained by a known technique of mixingan oxide powder containing Ni as a main component and lithium hydroxidefollowed by firing was used as a raw material.

More specifically, nickel sulfate, cobalt sulfate and sodium aluminatewere dissolved in water, a sodium hydroxide solution was further addedwith sufficient stirring, to produce a co-precipitate, i.e. anickel-cobalt-aluminum composite hydroxide co-precipitate so that themolar ratio of Ni, Co and Al satisfied Ni:Co:Al=82:15:3, theco-precipitate was washed with water and dried, and thereafter lithiumhydroxide-monohydrate was added to adjust the molar ratio so thatLi:(Ni+Co+Al)=103:100 was satisfied, thereby producing a precursor.

Next, the precursor was fired in an oxygen stream at 700° C. for 10hours, cooled to room temperature, and thereafter pulverized to obtain apowder of lithium-metal composite oxide made of lithium nickelaterepresented by the compositional formula: Li1.03Ni0.82Co0.15Al0.0302.

The lithium-metal composite oxide powder had an average particle size of12.4 μm and a specific surface area of 0.3 m2/g. The average particlesize was evaluated using a volume integrated average in the laserdiffraction light-scattering method, and the specific surface area wasevaluated using the BET method by nitrogen gas adsorption.

Next, a lithium tungstate powder to be mixed with the lithium-metalcomposite oxide powder was produced.

Lithium hydroxide/monohydrate (LiOH/H2O) and tungsten oxide (WO3) weremixed and reacted so that the molar ratio was 2:1. After completion ofthe reaction, the mixture was dried at 80° C. in the air atmosphere, toobtain a lithium tungstate powder.

After 1 g of the obtained lithium tungstate was added to 100 ml of waterhaving a pH of 12.5 at 25° C., and the resultant was stirred for 20minutes. After the stirring, solid-liquid separation was conducted byfiltration to remove the residue, and analysis by ICP emissionspectroscopy was conducted to determine the content of tungsten in thefiltrate. The amount of lithium tungstate dissolved was calculated fromthe analysis value to determine the solubility A, and the solubility Awas 1.2 g/L.

Hydrochloric acid was added to the filtrate after solid-liquidseparation while measuring the pH until the point of neutralizationemerged. When the state of a compound of lithium dissolved at theneutralization point was evaluated, 90 mol % or more of the lithiumtungstate was confirmed to correspond to Li2WO4.

Further, the crystal structure of the obtained lithium tungstate powderwas analyzed by the X-ray diffraction method (XRD), and the obtainedlithium tungstate powder was concluded to correspond to (Li2WO4)7(H2O)4.

75 g of a lithium-metal composite oxide powder as a raw material wasimmersed in 100 ml of pure water, and 0.29 g of the produced lithiumtungstate powder was added and stirred to be thus sufficient mixed, andthe lithium-metal composite oxide powder was washed with water at thesame time. The concentration of the slurry which was washed with water,corresponding to the solid-liquid ratio at the time of addition of W,was 750 g/L. After water washing, solid-liquid separation was performedthereon by filtration using a Buchner funnel, to obtain a mixture of thetungsten compound powder and the lithium-metal composite oxide powder.

The obtained mixture was put into a firing container made of SUS, thetemperature was raised in a vacuum atmosphere at a rate of temperatureincrease of 2.8° C./minute up to 210° C., for heat treatment at 210° C.for 13 hours, followed by cooling to room temperature in the furnace.

After cooling in the furnace, finally, a sieve with a mesh opening of 38μm was applied for deagglomeration, to obtain a positive electrodeactive material having a compound containing W and Li on the surface ofthe primary particles.

As a result of analyzing the tungsten content and the ratio Li/Me1 inthe obtained positive electrode active material by the ICP method, thecomposition was confirmed to be such that the tungsten content was 0.5at % with respect to the total number of atoms of Ni, Co, and M (Me1),and the Li/Me1 was 0.994.

[Morphological Analysis of Compound Containing Lithium and Tungsten]

The obtained positive electrode active material was embedded into aresin, and processing was performed thereon so that cross sectionobservation could be performed. The cross section of the resultant wasobserved by SEM at 5000-fold magnification, and it was confirmed thatthe resultant was composed of primary particles and secondary particlesformed by aggregation of the primary particles, and fine particles ofthe compound containing lithium and tungsten were formed on the surfaceof the primary particles, and the fine particles had a particle size of20 to 150 nm. Further, the void content of the secondary particles basedon the above image analysis, determined by the observation, was 0.51%.

Further, the obtained positive electrode active material was embeddedinto a resin so that the cross section of the secondary particles couldbe observed by a transmission electron microscope (TEM). Thereafter, thecross section of the secondary particles was observed by TEM and voidsbetween the primary particles at 25 points present in the secondaryparticles was analyzed by EDX, and as a result, W was detected at 84% ofthe number of the voids analyzed.

Further, the vicinity of the surface of the primary particles wasobserved by TEM, and it was confirmed that a coating thin film of thecompound containing lithium and tungsten with a film thickness of 2 to85 nm was formed on the surface of the primary particles, and thecompound was lithium tungstate.

Further, the state of lithium tungstate in the obtained positiveelectrode active material was evaluated by titrating Li eluted from thepositive electrode active material.

Pure water was added to the obtained positive electrode active material,the resultant mixture was stirred for a certain time. Hydrochloric acidwas added to the filtrate while measuring the pH of the filtrateobtained by filtration until the point of neutralization emerged. Whenthe state of a compound containing lithium eluted at the neutralizationpoint was evaluated, the presence of Li4WO5 and Li2WO4 was confirmed inthe lithium tungstate, and the proportion of Li4WO5 contained therein,as calculated, was 83 mol %.

[Evaluation of Battery]

The battery characteristics of the coin type battery 1 shown in FIG. 4having a positive electrode produced using the obtained positiveelectrode active material were evaluated. The positive electroderesistance was shown as a relative value, taking the evaluation value ofExample 1 as 100.

The initial discharge capacity was 204.6 mAh/g.

Hereinafter, for Examples 2 to 3 and Comparative Examples 1 to 2, onlymaterials and conditions changed from those in Example 1 above areshown. Further, the evaluation values of the initial discharge capacityand the positive electrode resistance of Examples 1 to 3 and ComparativeExamples 1 to 2 are shown in Table 1

Example 2

A positive electrode active material for nonaqueous electrolytesecondary batteries was obtained and also evaluated in the sameconditions as in Example 1 except that 0.12 g of lithium tungstate wasadded. The results are shown in Table 1.

Example 3

A positive electrode active material for nonaqueous electrolytesecondary batteries was obtained and also evaluated in the sameconditions as in Example 1 except that 0.35 g of lithium tungstate wasadded. The results are shown in Table 1.

Comparative Example 1

1.4 g of WO3 was added into an aqueous solution in which 0.6 g of LiOHwas dissolved in 5 ml of pure water, followed by stirring, to obtain analkaline solution (W) containing tungsten.

A positive electrode active material for nonaqueous electrolytesecondary batteries was obtained and also evaluated in the sameconditions as in Example 1 except that no lithium tungstate was added inwater washing and the alkaline solution (W) was added and mixed aftersolid-liquid separation. The results are shown in Table 1. The amount ofa liquid in addition of W (addition of the alkaline solution) wasdefined as the total of water remaining after solid-liquid separationand the alkaline solution, and the solid-liquid ratio was calculated.

Comparative Example 2

A positive electrode active material for nonaqueous electrolytesecondary batteries was obtained and also evaluated in the sameconditions as in Example 1 except that no lithium tungstate was added inwater washing and water washing was performed by pure water. The resultsare shown in Table 1.

Conventional Example

1.632 parts by weight of ammonium paratungstate ((NH4)10W12O41.5H2O) wasadded to 100 parts by weight of the lithium-metal composite oxide powderused in Example 1, and sufficiently mixed in a mortar to provide amixture, and the mixture was fired in an oxygen stream at 700° C. for 4hours and cooled to room temperature, and thereafter taken out andpulverized, to produce a positive electrode active material ofConventional Example.

The obtained positive electrode active material was used, and evaluatedas in Example 1. The results are shown in Table 1.

TABLE 1 W concentration Compound on surface of primary particles inpositive Solid-liquid Number of Initial electrode ratio in Heat voidswith Particle Film discharge Positive active material addition of Wtreatment W detected size thickness capacity electrode [at %] [g/L]conditions [%] Form [nm] [nm] [mAh/g] resistance Example 1 0.5 750 210°C. × 84 Thin film + 20-150 2-85 204.6 100 13 hr fine particles Example 20.2 750 210° C. × 72 Thin film — 1-80 205.5 116 13 hr Example 3 0.6 750210° C. × 88 Thin film + 30-170 2-90 200.0 108 13 hr fine particlesComparative 0.5 6590 210° C. × 41 Thin film + 20-210  2-105 197.9 137Example 1 13 hr fine particles Comparative 0 — 210° C. × — — — — 198.2235 Example 2 13 hr Conventional Li_(1.03)Ni_(0.77)Co_(0.20)Al_(0.03)O₂/— — — — 179.9 218 Example Ammonium paratungstate[Evaluation]

As is obvious from Table 1, the composite hydroxide particles and thepositive electrode active materials of Examples 1 to 3 were producedaccording to the present invention and therefore formed batteries havinghigh initial discharge capacity and low positive electrode resistance ascompared with Conventional Example, and excellent characteristics.

Further, an example of the cross-sectional observation results of thepositive electrode active material obtained in the examples of thepresent invention by TEM is shown in FIG. 2 and FIG. 3, where it wasconfirmed that the obtained positive electrode active material wasconstituted by primary particles and secondary particles formed byaggregation of the primary particles, and the compound containingtungsten and lithium was formed on the surface of the primary particles.

Meanwhile, those of Comparative Example 1, in which the amount oftungsten with respect to the number of atoms of Ni, Co and M containedin the lithium-metal composite oxide powder was almost the same as inExample 1, were obtained by a method where an alkaline solutioncontaining tungsten was added and mixed after water washing, and weredeteriorated in dispersion uniformity as compared with Examples andcaused the compound containing tungsten and lithium to be unevenlyformed, thereby resulting in an increase in positive electroderesistance as compared with Example 1.

In Comparative Example 2, since the compound containing W and Liaccording to the present invention was not formed on the surface of theprimary particles, the positive electrode resistance was considerablyhigh and it was difficult to meet the requirement to enhance the power.

In Conventional Example, since the mixing with the solid tungstencompound was performed, W was not sufficiently dispersed and Li was notsupplied into the compound, resulting in considerably high positiveelectrode resistance.

It can be confirmed from the above results that a nonaqueous electrolytesecondary battery using the positive electrode active material obtainedby the present invention has high initial discharge capacity and alsolow positive electrode resistance and forms a battery having excellentcharacteristics.

The nonaqueous electrolyte secondary battery of the present invention issuitable for power sources of small portable electronic devices (such aslaptop personal computers and mobile phone terminals) that constantlyrequire high capacity and is suitable for batteries for electric carsthat require high power.

Further, the nonaqueous electrolyte secondary battery of the presentinvention has excellent safety and allows size reduction and powerenhancement, and therefore it is suitable as a power source for electriccars where there is a restriction on the mounting space. The presentinvention can be used not only as a power source for electric cars whichare purely driven by electric energy but also as a power source forso-called hybrid vehicles that is used in combination with a combustionengine such as a gasoline engine and a diesel engine.

REFERENCE SIGNS LIST

-   1: Coin type battery-   2: Case-   2 a: Positive electrode can-   2 b: Negative electrode can-   2 c: Gasket-   3: Electrode-   3 a: Positive electrode-   3 b: Negative electrode-   3 c: Separator

What is claimed is:
 1. A method for producing a positive electrodeactive material for nonaqueous electrolyte secondary batteries,comprising: a mixing step of adding a tungsten compound powder having asolubility A adjusted to 2.0 g/L or less to a lithium-metal compositeoxide powder so as to reach a solid-liquid ratio of 500 to 1500 g per 1L and stirring them in water washing of the lithium-metal compositeoxide powder to form a slurry having a slurry temperature of 15 to 35°C. and a pH value of the slurry of 11.5 to 13.5, the solubility A beingdetermined by stirring the tungsten compound in water having a pH of12.5 at 25° C. for 20 minutes, the lithium-metal composite oxide powderbeing represented by a general formula: Li_(c)Ni_(1-x-y)Co_(x)M_(y)O₂,where 0≤x≤0.35, 0≤y≤0.35, and 0.97≤c≤1.25, are satisfied, and M is atleast one element selected from Mn, V, Mg, Mo, Nb, Ti and Al, and beingcomposed of primary particles and secondary particles formed byaggregation of the primary particles, followed by solid-liquidseparation, to thereby obtain a tungsten-containing mixture with thetungsten compound dispersed in the lithium-metal composite oxide powder;and a heat-treating step of heat-treating the tungsten-containingmixture obtained in the mixing step at a temperature of 100 to 600° C.in an oxygen atmosphere or in a vacuum atmosphere to uniformly dispersetungsten on a surface of the primary particles of the lithium-metalcomposite oxide powder, and thereafter form a compound containingtungsten and lithium from the tungsten dispersed uniformly on thesurface of the primary particles and lithium in the tungsten-containingmixture, on the surface of the primary particles of the lithium-metalcomposite oxide powder.
 2. The method for producing a positive electrodeactive material for nonaqueous electrolyte secondary batteries accordingto claim 1, wherein the tungsten compound added in the mixing step islithium tungstate.
 3. The method for producing a positive electrodeactive material for nonaqueous electrolyte secondary batteries accordingto claim 2, wherein the lithium tungstate is obtained by a reaction oflithium hydroxide and a tungsten compound.
 4. The method for producing apositive electrode active material for nonaqueous electrolyte secondarybatteries according to claim 1, wherein an amount of tungsten containedin the tungsten-containing mixture is 0.05 to 3.0 at % with respect tothe total number of atoms of Ni, Co and M contained in the lithium-metalcomposite oxide powder mixed.
 5. The method for producing a positiveelectrode active material for nonaqueous electrolyte secondary batteriesaccording to claim 1, wherein the tungsten compound added in the mixingstep comprises 80% or more of (Li₂WO₄)₇(H₂O)₄.
 6. The method forproducing a positive electrode active material for nonaqueouselectrolyte secondary batteries according to claim 1, wherein atemperature in the water washing in the mixing step is 40° C. or less.